/Engine/Source/Runtime/Engine/Classes/Materials/MaterialExpressionStrata.h
A starta material is a tree with FrontMateiral being its root and BSDF being leaves, with operators in the middle. This recursively parse nodes outputing strata material in order to gather the maximum distance to any leaves. This is used to drive the bottom up order processing of those nodes.
virtual [FStrataOperator](API\Runtime\Engine\Rendering\FStrataOperator) * StrataGenerateMaterialTopologyTree ( class [FMaterialCompiler](API\Runtime\Engine\FMaterialCompiler) * Compiler, class [UMaterialExpression](API\Runtime\Engine\Materials\UMaterialExpression) * Parent, int32 OutputIndex )